It is certainly hard to go from being the number one priority to feeling pushed aside and de-prioritized for the new shiny one. I wasn't able to cope with that in a past relationship myself.
Is he willing to talk to you about how you are feeling? Is she? Could the three of you talk together?
Natja, I was applauding at your post. I too do not see a point to this forum unless we are here to try to help instead of attack.